1. Field
The embodiments discussed herein are related to a method and an apparatus for transmitting data from an asynchronous network via transmitting apparatuses of a synchronous network.
2. Description of the Related Art
Owing to the growth of, for example, the Internet, Local Area Network (LAN) services such as Ethernet (registered trademark) are becoming extremely active. Communication carriers seek to expand service menus, improve communication quality, and provide inexpensive wideband services. Furthermore, communication carriers, in general, build systems by utilizing existing network facilities as much as possible.
In view of the above, PPP (Point-to-Point Protocol) over SONET/SDH is widely used as a method for transmitting packet data of an asynchronous network (e.g., LAN using Ethernet (registered trademark) by using a highly reliable synchronous network that has been widely used from the past (e.g., SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy)). It is to be noted that, in the description of the specification of the present application, a Wide Area Network (WAN) and a Metropolitan Area Network (MAN) are also included in the term “LAN (Local Area Network)”.
FIG. 1 is a schematic diagram illustrating a network configuration using the PPP over SDH method. In FIG. 1, PPP over SDH transmitting apparatuses (hereinafter referred to as “transmitting apparatus”) 1, 2 have a function of mapping LAN frames of data from client apparatuses 3, 4 into SDH data areas and transmitting the mapped data by using a wideband SDH transmission network.
FIG. 2 is a block diagram illustrating a PPP over SDH transmitting apparatus according to a related art example. FIG. 3 illustrates a format of a PPP frame. FIG. 4 is a schematic diagram for describing an example of a byte stuffing method.
A PPP over SDH transmitting apparatus (see, for example, IETF RFC 2615) or a PPP over SONET transmitting apparatus encapsulates packet data from an external apparatus (client apparatus) connected to a SONET/SDH network into information fields of HDLC (High Level Data Link Control) like framing (see, for example, IETF RFC 1662) in frame units, maps the encapsulated data into a payload of SONET/SDH frames, and transmits the mapped data.
As illustrated in FIG. 3, in HDLC like framing, the gap parts between frames are filled with flags (in this example, 0x7E indicates a flag where “0x” indicates hexadecimal) so that the data between the flags can be recognized as a PPP frame.
Thus, in a case where data 0x7E (data equal to a flag) are included in a PPP frame, the PPP frame would be erroneously recognized. Accordingly, in a case where a PPP frame includes data 0x7E, a byte stuffing part (as illustrated in, for example, 13) converts the data 0x7E into data 0x7DE (in this example, 7D indicates an escape code) as illustrated in FIG. 4. Likewise, in a case where data 0x7D is included in a PPP frame, the PPP frame would be erroneously recognized as an escape code. Accordingly, in a case where a PPP frame includes data 0x7D, the data 0x7D is converted into data 0x7D5D. This process is referred to as byte stuffing.
In FIG. 2, LAN packet data supplied from an external apparatus are supplied to a transmitting apparatus 10. The supplied data are encapsulated in an information field of a PPP frame (HDLC like framing) by a PPP encapsulating part 11. The encapsulated data are added with a Frame Check Sequence (FCS) by an FCS generating part 12. Then, the byte stuffing part 13 performs a byte stuffing process on the encapsulated data. Then, the byte-stuffed encapsulated data are scrambled by a scrambling part 14. Then, an SDH framing part 15 maps the scrambled encapsulated data into SDH frames and fills gap parts with flags, to thereby transmit the SDH frames to an SDH transmission network 16.
The SDH frames transmitted from the SDH transmission network 16 are received by a receiving apparatus 20. In the receiving apparatus 20, flags provided between the SDH frames are removed by an SDH framing part 21. Then, a descrambling part 22 descrambles the SDH frames. Then, a byte de-stuffing part 23 de-stuffs the descrambled SDH frames to thereby return the descrambled SDH frames to a state being subject to the byte stuffing process. Then, after FCS detection is performed on the destuffed SDH frames, a PPP decapsulating part 25 decapsulates the SDH frames, to thereby obtain the LAN packet data. Then, the PPP decapsulating part 25 transmits the LAN packet data to another external apparatus.
In order to control the transmission amount of a PPP frame, Japanese Laid-Open Patent Publication No. 2006-135754 discloses a device that detects the amount of data received from other apparatuses and uses fixed stuff for storing a PPP frame in a case where the received data exceeds a threshold.
Further, as another related art example, Japanese Laid-Open Patent Publication No. 2001-86156 discloses a system where fixed values in an address field and a control field of a PPP frame are used as identification code values corresponding to various information.
Further, Japanese Laid-Open Patent Publication No. 5-292050 discloses a system where a part of a synchronization pattern of an SDH frame is left unscrambled so that degradation of bit error rate (e.g., pattern jitter) can be prevented by inverting each byte of the synchronization pattern.
In HDLC like framing where 0x7E (same code as flag) and 0x7D (same code as escape code) exists in the original data, one byte of data is converted into two bytes of data including an escape code, to thereby increase the amount of data flowing in the SDH transmission path. In a worst case (a case where all of the original data are 7E/7D), the converted data becomes twice its original data amount.
Particularly, since the transmitting apparatus 10 is unable to control the client's data mapped into the information field that forms a large part of the PPP frame, data may become lost due to lack of bandwidth unless a bandwidth doubling the client's data is prepared in the SONET/SDH. Accordingly, it is desired to operate the client's data bandwidth by preparing a bandwidth having a margin added to the client's data bandwidth in the SONET/SDH transmission path. This, however, leads to decrease of line utilization.